Active antenna arrays comprise a plurality of transceiver modules for receiving and transmitting signals. To operate the active antenna array in an efficient manner, transmitter paths to the transceiver modules have to be calibrated in order so that the transmitter paths work together in a coherent manner. In other words, magnitude and phase of individual signals on the transmitter paths have to be synchronized to ensure that the individual signals on the transmitter paths are coherently combined and also to allow accurate signal processing means, such as beam-forming, tilting, or delay diversity techniques.
To be able to synchronize the plurality of the transmitter paths, the magnitude deviations and the phase deviations between the transmitter paths have to be determined in order to compensate for the magnitude deviations and the phase deviations of the individual signals by signal processing means. Some of the magnitude deviations and the phase deviations are induced by deterministic effects (e.g. different cable lengths) and may be calibrated offline during manufacturing. However, in most antenna arrays, there are time-varying statistical effects which additionally require an online calibration technique to compensate for such time-varying statistical effects.
The calibration of the transmitter paths is an element in constructing active antenna arrays. There are several methods known in the literature for performing the calibration of the transmitter path. Two different types of calibration methods may be distinguished: “blind” calibration methods and “pilot-based” calibration methods. Blind calibration methods estimate the magnitude and phase deviations by observing and comparing signals at the input and the output of the antenna system. Pilot-based calibration methods use known auxiliary signals to measure any deviations between the transmitter paths.
A common pilot-based calibration method injects a calibration signal into the so-called wanted signal. The calibration signal can be detected in the wanted signal and can be uniquely attributed to a particular one of the transmitter paths. The calibration needs to be done in such a manner that the calibration signal does not significantly interfere with the wanted signal. In order to do this, the calibration signal should be of low power. On the other hand, to achieve a high degree of accuracy for the calibration, the calibration signal has to carry a significant amount of energy. In order to solve this conflict, several known calibration methods use some kind of low-power pseudo-noise sequences which spread the energy of the calibration signal over a large period of time and a large frequency band. However, if the power of the calibration signal is smaller than the power of the wanted signal by several orders of magnitude, the required processing gain requires such long pseudo-noise sequences which may render the time period of the calibration process unfeasibly long.
Blind calibration methods work without requiring an interfering pilot signal (or calibration signal). Blind calibration methods observe the wanted signal at the input and at the output of the antenna arrays and use the difference between the input signal and the output signal to adapt a model of the active antenna array which is to be calibrated. It has been found, however, that such blind calibration methods may tend to become instable or inaccurate for larger magnitude and phase deviations. Thus blind calibration methods are usually only used in systems which are already substantially pre-calibrated.
A number of prior art patents are known in which calibration methods are discussed. For example, U.S. Pat. No. 6,693,588 (Schlee, assigned to Siemens) discusses an electronically phase-controlled group antenna which is calibrated in radio communication systems using a reference point shared by all the reference signals. In the down-link procedure, reference signals which are distinguishable from one another are simultaneously transmitted by individual ones of the antenna elements of the antenna array. The reference signals are separated after reception at the shared reference point.
U.S. Pat. No. 7,102,569 (Tan et. al., assigned to Da Tang Mobile Communications Equipment, Bej Jing) teaches a method for establishing transmission and receiving compensation coefficients for each one of the antenna elements relative to a calibration antenna element.
European Patent Application No. 1 178 562 (Ericsson) teaches a method and a system for calibrating the reception and the transmission of an antenna array for use in a cellular communication system. The calibration of the reception of the antenna array is performed by injecting a single calibration signal into each of the plurality of the receiving antenna sections in parallel. The signals are collected after having passed receiving components which might distort the phase and the amplitude of the signals. Correction factors are generated and are applied to receive signals. The calibration of the transmission of the antenna array is performed by generating a single calibration signal into each of the plurality of the transmitting antenna sections. The signals are collected and correction factors are generated and applied to signals.